This invention relates to a method and apparatus for removing sulfur from hydrocarbon-containing fluid streams. In another aspect, the invention concerns an improved system for transferring solid sorbent particulates between vessels in a hydrocarbon desulfurization unit.
Hydrocarbon-containing fluids such as gasoline and diesel fuels typically contain a quantity of sulfur. High levels of sulfurs in such automotive fuels is undesirable because oxides of sulfur present in automotive exhaust may irreversibly poison noble metal catalysts employed in automobile catalytic converters. Emissions from such poisoned catalytic converters may contain high levels of non-combusted hydrocarbons, oxides of nitrogen, and/or carbon monoxide, which, when catalyzed by sunlight, form ground level ozone, more commonly referred to as smog.
Much of the sulfur present in the final blend of most gasolines originates from a gasoline blending component commonly known as “cracked-gasoline.” Thus, reduction of sulfur levels in cracked-gasoline will inherently serve to reduce sulfur levels in most gasolines, such as, automobile gasolines, racing gasolines, aviation gasolines, boat gasolines, and the like.
Many conventional processes exist for removing sulfur from cracked-gasoline. However, most conventional sulfur removal processes, such as hydrodesulfurization, tend to saturate olefins and aromatics in the cracked-gasoline and thereby reduce its octane number (both research and motor octane number). Thus, there is a need for a process wherein desulfurization of cracked-gasoline is achieved while the octane number is maintained.
In addition to the need for removing sulfur from cracked-gasoline, there is also a need to reduce the sulfur content in diesel fuel. In removing sulfur from diesel fuel by hydrodesulfurization, the cetane is improved but there is a large cost in hydrogen consumption. Such hydrogen is consumed by both hydrodesulfurization and aromatic hydrogenation reactions. Thus, there is a need for a process wherein desulfurization of diesel fuel is achieved without significant consumption of hydrogen so as to provide a more economical desulfurization process.
Recently, improved desulfurization techniques employing regenerable solid sorbents have been developed. Such regenerable sorbents typically include a metal oxide component (e.g., ZnO) and a promoter metal component (e.g., Ni). When contacted with a sulfur-containing hydrocarbon fluid (e.g., cracked-gasoline or diesel fuel) at elevated temperature and pressure, the promoter metal and metal oxide components of the regenerable sorbent cooperate to remove sulfur from the hydrocarbon fluid and store the removed sulfur on/in the sorbent via the conversion of at least a portion of the metal oxide component (e.g., ZnO) to a metal sulfide (e.g., ZnS). The resulting “sulfur-loaded” sorbent can then be regenerated by contacting the sulfur-loaded sorbent with an oxygen-containing stream at elevated temperature and reduced pressure. During such regeneration, at least a portion of the metal sulfide (e.g, ZnS) in the sulfur-loaded sorbent is returned to the metal oxide (e.g., ZnO) via reaction with the oxygen-containing regeneration stream, thereby providing a regenerated sorbent.
Traditionally, solid sorbent compositions used in hydrocarbon desulfurization processes have been agglomerates utilized in fixed bed applications. However, because fluidized bed reactors provide a number of advantages over fixed bed reactors, it is desirable to process hydrocarbon-containing fluids in fluidized bed reactors. One significant advantage of using fluidized bed reactors in desulfurization systems employing regenerable solid sorbents is the ability to continuously regenerate the solid sorbent particulates after they have become “loaded” with sulfur. Such regeneration can be performed by continuously withdrawing sulfur-loaded sorbent particulates from the fluidized bed desulfurization reactor and transferring the sulfur-loaded sorbent particulates to a separate regeneration vessel for contacting with the oxygen-containing regeneration stream. When the sulfur-loaded sorbent particulates are transferred from the desulfurization reactor to the regenerator, they are transferred from a high temperature, high pressure, hydrocarbon environment (in the reactor) to a high temperature, low pressure, oxygen environment (in the regenerator). The different pressures and atmospheres in the reactor and regenerator present a variety of challenges when continuously withdrawing and regenerating sulfur-loaded sorbent particulates from the reactor. For example, the pressure differential between the reactor and regenerator can make it difficult to maintain the proper pressures in the reactor and regenerator while continuously transferring sulfur-loaded solid particulates from the reactor to the regenerator. Further, safety concerns require that the hydrocarbon environment of the reactor and the oxygen environment of the regenerator remain substantially isolated from one another in order to prevent combustion of hydrocarbons from the reactor when exposed to oxygen from the regenerator. Such isolation of the hydrocarbon environment in the reactor from the oxygen environment in the regenerator can be difficult to maintain during continuous transfer of sulfur-loaded sorbent particulates from the reactor to the regenerator.